1. Field of the Invention
The present invention pertains to an image reader used in an original document reading unit of a copying machine, scanner, etc.
2. Description of the Related Art
In a copying machine using the mirror scan method, for example, an original document is placed on the platen glass of the original document reading unit of the copying machine, and by moving a scanner located below the platen glass parallel to the surface of the original document, the image of the original document is optically read.
If the speed of movement of the scanner (scanning speed) and the timing for the commencement of reading are unstable during this image reading, errors occur in the reproduced image and high-quality image formation cannot be achieved. Therefore, a stepping motor is used as the drive source for the scanner moving mechanism, and the control of the scanner's scanning speed, reading position, etc., is accurately performed using pulse control.
This type of stepping motor is driven to rotate by exciting groups of multiple stator coils (generally called "phases") located around the rotor through the impression of rectangular fixed-current drive pulses (full-step pulses), wherein these excited phases are alternated in a sequential manner. Methods of excitation include the single-phase excitation method in which only one phase is excited at any one time, the two-phase excitation method in which two phases are excited at any one time, or the alternating-phase excitation method in which these two methods are alternated.
However, in the drive control (hereinafter termed "rectangular wave driving") of the stepping motor using the full-step pulses, the drive current spikes abruptly each time the excited phases are alternated, thus causing the magnetic flux distribution inside the motor to change suddenly producing fluctuations in the drive torque (torque ripple), which cause unsmooth rotation leading to noise and vibration.
When vibration occurs, it is transmitted to the scanner via the power transmission mechanism, negatively affecting the accuracy of the image reading and causing blurry images.
To prevent this problem, U.S. Pat. No. 5,583,620 discloses a method to control the rotation of a stepping motor by dividing the rectangular drive pulses, that are sequentially impressed to the phases, into multiple steps so that the rectangular pulse will resemble a sine curve. This driving method, which is generally called the microstep driving method, is advantageous in that changes in the drive current at the rising and the falling of the pulse are mild, producing little fluctuation in the magnetic flux distribution. Therefore, torque ripple does not occur easily, allowing the rotor to rotate smoothly. Consequently, when the stepping motor is driven, noise and vibration are reduced making improved image reading accuracy possible.
On the other hand, with the popularization of automatic original document feeders that can continuously send multiple original documents to the image reading unit, it has become widely desired to increase both the efficiency of reading an original document and the rate of reproduction (hereinafter "CPM" or "copies per minute") of the copying machine. To meet this need, it is necessary to return the scanner as quickly as possible to the initial scan position after the scanner has read an original document.
In order to increase the return speed as described above, a high-torque stepping motor must be used, but in this case a new problem arises, i.e., slider vibration occurs in the scanner when scanning an original document.
In other words, during scanning of an original document, because the scanning speed is not particularly high, the torque necessary for the stepping motor can only be a fraction of that necessary when the scanner is returned to the initial scan position. However, by using the high-torque stepping motor described above, the balance between the drive torque of the stepping motor during scanning of an original document and the load torque during scanner movement is destroyed, and the scanner vibrates along the rails that slidably support the scanner (this phenomenon is hereinafter termed "slider vibration"), which de-synchronizes the scanning process and reduces the quality of the reproduced image.
In a copying machine offering different copy magnifications, the scanning speed at the largest magnification is extremely slow, this causes a disparity in the torque balance to be more pronounced, worsening the slider vibration, resulting in marked deterioration in the quality of the reproduced image.
When driving control is performed by means of the microstep driving method described above, the drive torque control capacity is limited to the number of microsteps into which the drive pulses are divided is increased, thereby increasing the resolution. Even where drive torque control by means of the microstep driving is possible to some extent, in a mechanism such as a copying machine in which a wide magnification range and minute variations in magnification are desired, the scanning speeds are set in correspondingly precise increments, thereby changing the load torque becomes complex, which not only requires that a CPU used for drive control be capable of very fast processing speeds and extremely high resolution, but also increases the amount of memory needed for storing the control programs, which increases cost considerably.